Lithographic apparatus comprises a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Lithographic apparatus may be of the transmissive type, where radiation is passed through a mask to generate the pattern, or of the reflective type, where radiation is reflected from the mask to generate the pattern. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Developments in the field of lithography for improving projection image quality have demonstrated the need to improve certain optical characteristics of the radiation beam. Generally these optical characteristics comprise the homogeneity or uniformity of the beam, the size and/or shape of the beam, the pointing direction of the beam, the position of the beam with respect to the optical axis, and the divergence of the beam, all of which will be referred to collectively hereafter as “optical characteristics” of the beam. Currently only limited options are available to control such beam optical characteristics in lithographic apparatus. In particular it is possible for the optics of the illumination system of such apparatus to be adjusted by means of adjusting screws during installation based upon information received from a beam analysis module.
The characteristics of the radiation beam have an effect on the angular distribution delivered by the illumination system. In the current state of the art, the angular distribution is changed by changing the positions of optical elements within the illumination system in a predetermined way. However the optical characteristics of the input radiation beam are not taken into account in the control of the angular distribution.
U.S. Pat. No. 6,870,603 discloses lithographic apparatus incorporating a tiltable steering mirror for directing the radiation beam to a projection system, and for adjusting lateral displacement of the beam in X- and Y-directions relative to an aperture formed by a diffractive optical element within the projection system. A processor is provided for receiving information relating to the tilt of the mirror, as well as information relating to the beam intensity measured by a detector within the illumination system. By varying the tilt angle of the mirror and making corresponding measurements of the beam intensity, it is possible to determine the beam size. Furthermore the beam size can be adjusted by means of an optical conditioning unit through which the beam is passed before being supplied to the mirror. However, it is not practicable to use such an arrangement for adjusting the size and divergence of the beam to match the beam to a target pupil shape.
U.S. Pat. No. 5,121,160 discloses exposure apparatus incorporating a beam expander and a zoom expander for adjusting the shape and size of a radiation beam supplied to a projection system. Furthermore a photodetector is provided on the support for the substrate to be exposed to provide measurements of the illuminance at the image surface of the projection system. Such an arrangement does not enable adjustment of the beam on the basis of the detected size and divergence of the beam.
U.S. Pat. No. 5,724,122 and U.S. Pat. No. 5,991,016 also disclose arrangements for adjustment of the radiation beam in an exposure system, but such arrangements again to do enable adjustment of the beam in dependence on the detected size and divergence of the beam.